Independent control of limb position and contact forces in cat posture

1. It has previously been demonstrated that a set of geometric and kinetic parameters are invariant in cats standing at their preferred interfoot distance and weight distribution. Thus the length and the angle of orientation relative to the vertical of each limb axis remain approximately constant when the supporting platform is tilted in the sagittal plane. The direction of the tangential contact forces is similarly constrained in response to horizontal translations. The main aim of the present study is to assess whether or not the control of limb position is independent of the control of the contact forces at the feet. To this end we have examined cat posture under a number of different conditions expressly designed to increase the range of postural variability. We considered that if the specification of limb position is a mere byproduct of the neural control of contact forces (or vice versa), geometric and kinetic parameters would covary interdependently. If instead limb position and contact forces are controlled in parallel and independently of each other, they will tend to follow different laws of variation. 2. Limb position and contact forces were measured in intact cats standing freely on a support platform. In a first series of experiments the pitch angle of the platform was randomly changed, as were the interfoot distance and head orientation. In another series of experiments cats were tilted in the presence of an external load tending to shift the weight distribution. The same load was applied in two different manners: 1) it made contact with a very limited surface of the body, and 2) it was attached by means of a long vest that made contact with most of the trunk and produced abnormal somesthesic cues to the body. 3. The range of different experimental conditions resulted in substantial trial-to-trial variations of the length and orientation of the axis of the limbs, as well as variations of the magnitude and orientation of the net contact forces. We found that the changes of the orientation of the contact force vector are uncorrelated with the corresponding changes of limb orientation, thus providing a first line of evidence in favor of the existence of a separate neural control of geometric and kinetic parameters. 4. Another line of evidence is provided by the specific form of the laws of variation of geometric parameters and tangential forces in different animals. Under normal (unloaded) conditions the values of the limb joint angles tend to covary linearly.(ABSTRACT TRUNCATED AT 400 WORDS)